1. Field of the Invention
This invention relates to apparatus for detecting water penetration, for example, into a power cable and a communication cable.
2. Prior Art
Generally, a power cable and a communication cable are installed in transmission line passages inside and outside the city and aee also installed in the bottom of the sea.
Heretofore, alarm wires (i.e., sensor) in hhe form of a bare wire, a paper-insulated wire or a plastics-insulated wire is mounted within the calle so as to extend therealong, and when water penetrates into the cable at a location, the alarm wires are short-circuited at that location for the purpose of detecting the water penetration. However, with such alarm wires, the location of the cable at which the water penetration has occureed can not be detected accurately and rapidly. In addition, such alarm wires have a relatively low sensitivity to the water penetration, and therefore it is necessary to increase the number of the cables joints in order to enhance the sensitivity. Further, the alarm wires which comprises a conductor of metal can not be used in those regions where electromagnetic induction is encountered.
In recent years, very long optical fiber cables are jointed together at their ends to provide a communication line, in which case the alarm wires are too short to be mounted within such an optical fiber cable over the entire length of the cable. Also, in the conventional communication cable and the optical cable, there is a risk that water tends to penetrate or intrude into the transmission line though the cable joint. Therefore, it has long been desired to provide a water penetration-detecting apparatus which is free from the above problems.